Re: [chrony-users] makestep in Chrony

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On Tue, May 15, 2018 at 05:53:25AM -0700, Stephen Satchell wrote:
> Both NTP and chrony assume that the base clock has an absolute accurate of
> 100 ppm or better, and a relative accuracy of 5 ppm or better.

There is no such assumption in chrony. It will work fine with any
frequency offset that still allows chrony to control the clock.

> Now, in a VM object, the base clock used for timekeeping is almost worthless
> because the error and jitter fall outside of those boundaries.  That's why I
> scream and yell every time one of my clients installs NTP into a virtual
> machine and calls it good.  You are almost guaranteed to have a false
> ticker.  I could tell you stories...

That depends on the clocksource. With some the clock may be very
unstable depending on what the host and other guests do. However, with
the tsc clocksource the system clock of the VM can be as good as the
host's. FWIW, I have some servers running on oversold VPSes and they
have no problems with the clock. Virtualized networking seems to be a
bigger problem. Some drivers (e.g. e1000) support SW timestamping and
that can help a lot.

Miroslav Lichvar

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